PVA and poly(meth)acrylic acid and partially hydrolyzed products thereof are water soluble polymers, being widely used for water adsorbents, thickeners, flocculants, dispersants and treatment agents for paper and textiles. Poly(meth)acrylic acid or its partially neutralized product can be formed into a film by casting from its solution and the film obtained has an excellent gas barrier property under dry conditions. However, since this film is strongly hydrophilic, the gas barrier property is remarkably lost under high humidity circumstances besides being readily dissolved in water. Accordingly, this film is not suitable for packaging of foodstuffs containing a large amount of moisture.
While films made of starch and its derivatives have a good oil resistance and oxygen gas barrier property, on the other hand, they have a drawback that their mechanical strength is weak and water resistance is poor. Starch is one of the natural polysaccharides available in plants, its constituents being composed of linear amylose in which glucose units are linked by .alpha.(1-4) bonds and high molecular weight amylopectin in which multiple short amylose chains are branched with each other via .alpha.(1-6) bonds. Starch and its derivatives include, in addition to crude starch, processed starch, e.g., physically modified starch, such as purified starch, chemically modified starch prepared by hydrolysis of starch with an acid or enzyme, or by heating to enhance solubility in cold water, graft-modified starch obtained by graft polymerization with monomers such as acrylamide, acrylic acid, vinyl acetate or acrylonitrile. These modified starch derivatives are hydrophilic polymers, similarly to poly(meth)acrylic acid and, therefore, are used in a wide range of industrial fields, including not only in food industries but also for water adsorbents, thickeners, flocculants, dispersants and processing agents for paper and textiles by taking advantage of their hydrophilic property. The substance having good water solubility of these starch derivatives can be readily formed into a film by casting from an aqueous solution. However, films made of starch derivatives are not suitable for use in packaging of food stuffs because the films are so strongly hydrophilic that their oxygen gas barrier property is remarkably lost under high humidity conditions.
When PVA films are used for the purpose where a practical oxygen gas barrier property is required, it was attempted to diminish the effect of humidity as much as possible by forming the film into a laminated film of PVA films with other films having two or more of multiple layers. However, since forming into laminated films is not a sufficient countermeasure with respect to moisture resistance and water resistance, it is desired to endow the PVA film itself with water resistance along with sufficient oxygen gas barrier property in a high humidity environment.
U.S. Pat. No. 2,169,250 proposes a method for making films and textiles, formed from a mixed aqueous solution of PVA and a polycarboxylic acid followed by heating, insoluble in water by forming a cross-linked structure by allowing hydroxyl groups of PVA to react with carboxylic groups. Several methods for producing films and sheets from a mixture of starch and its derivatives and a variety of thermoplastic resins have been also proposed. A biodegradable film comprising PVA polymers and starch derivatives is described in Japanese Unexamined Patent Publication No. 4-100913 and Japanese Unexamined Patent Publication No. 4-114044. Japanese Unexamined Patent Publication No. 4-114043 discloses a water resistant composition comprising PVA polymers and polysaccharides and a film made of the composition. According to the examination by the inventors of the present invention, however, more improvements with respect to the oxygen gas barrier property in a high humidity environment are desired for practical purposes in the packaging of foodstuffs whose ingredients contain a lot of moisture.
The investigators of the present invention have proposed, in Japanese Unexamined Patent Publication No. 7-102083, a film formed of a mixture of polyvinyl alcohol and partially neutralized product of poly(meth)acrylic acid having an excellent water resistance and oxygen gas barrier property. Japanese Unexamined Patent Publication No. 7-165942 has proposed a film formed of a mixture of a poly(meth)acrylic acid polymer, selected from poly(meth)acrylic acid and partially neutralized products of poly(meth)acrylic acid, and saccharide, wherein the film is excellent in water resistance and oxygen gas barrier property.
Japanese Unexamined Patent Publication No. 7-205379 has proposed a multilayer gas barrier laminated film, containing a film insoluble in water and excellent in oxygen gas barrier property as a gas barrier layer formed of a mixture containing vinyl alcohol and poly(meth)acrylic acid or a partially neutralized product of poly(meth)acrylic acid, and a method for producing the same; while Japanese Unexamined Patent Publication No. 7-251485 has proposed a multilayer gas barrier laminated film, containing a film excellent in water resistance and oxygen gas barrier property as a gas barrier layer, formed of a mixture containing poly(meth)acrylic acid and/or a partially neutralized product of poly(meth)acrylic acid and saccharide, and a method for producing the same. The investigators have studied to effectively make use of water resistance and gas barrier property involved in these films.
The object of the present invention is to provide a container for retort packaging being resistant to high temperature steam and hot water (hot-water resistance) and being excellent in gas barrier property, wherein the oxygen gas barrier property is not changed after being subjected to a hot-water treatment or is not decreased below the level before the hot-water treatment, and a retort method using the same. Another object is to provide a specified resin composition and a gas barrier film comprising the same, and a method for producing the same.